Why does Apache have MaxClients and ServerLimit

It seems you are learning the hard way that PHP is a memory hog and not particularly scalable.

Some suggestions, in no particular order:

If you still suspect a memory leak set MaxRequestsPerChild to really low value.

Consider buying more memory, 360 megs is really not much nowadays.

Try finding an average size of httpd process by running ps or top and then setting MaxClients so that everything always fits in memory. Swapping is a death spiral, the slower you are processing the requests because of it, the more processes apache needs to fork, using even more memory.

If you laod php as a module in apache it gets loaded for every request, be it a script or a static file (an image or css or .js or whatnot). Consider serving static content from a separate server or using fastcgi or a reverse proxy like nginx to have apache serve only php to limit the number of fat php instances you have to keep in memory.

[For the benefit of others stumbling across this older question ... ]

Quick Answer:

Check your KeepAlive settings in your apache2.conf or httpd.conf file. Set your KeepAliveTimeout between 2-5 seconds.

Details:

I've found that by default Apache's KeepAlive is on and the KeepAliveTimeout is set to 15 seconds. That will mean that a single user's page hit will cause the server to wait for that same user to request another page/resource for 15 seconds, before it gives up and handles someone else's request.

This setup is VERY useful when a user requests the initial index.html file, and then will request the linked CSS, javascript, and image files a second or two later. However, modern computers and network/internet connections mean that a browser is typically asking for the linked resources in less than 2 seconds. Apache will serve those subsequent pages and then wait another 15 seconds in case that user wants something else. This is highly inefficient in a high traffic environment.

If you're receiving 15 unique connections per second, and each connection stays alive for 15 seconds... I'm sure you can see how things are going to get bunched up pretty severely pretty quick. You'll have 225 Apache processes spun-up, with 90+% of them completely idle, waiting for another page request on it's open connection.

I've seen a number of suggestions to set your KeepAliveTimeout to somewhere between 2 and 5 seconds. Myself, I've got some servers set as low as 2 and others as high as 5. I don't receive the same system slow downs when I get traffic spikes anymore.